U.S. Pat. No. 4,157,085 discloses a surgically implantable device for expanding skin and mucous tissue. The device comprises a partially collapsed sealed envelope formed from a material that is permeable to extracellular body fluids, and a material which establishes an osmotic potential across the envelope wall. As bodily fluid crosses the membrane to fill the envelope, the envelope expands, thereby expanding the adjacent tissue. The envelope material, which ranges from 0.001 to 0.020 inches in thickness, can be formed from such polymers as cellulose acetate, crosslinked polyvinyl alcohol, polyurethane, nylon, ethylene vinyl acetate copolymer, polyolefin, polyvinyl chloride, and silicone rubber. The material used to establish the osmotic potential can be, for instance, sodium chloride, calcium chloride, potassium sulphate, sodium acetate, ammonium phosphate, magnesium succinate, glucose, alanine, vinyl pyrrolidone, carboxymethylcellulose, urea, “blood by-products”, proteins, and/or dextran. The amount of expansion material used is selected depending on the desired speed of expansion of the envelope and the desired final volume thereof.
U.S. Pat. No. 5,005,591 points out that the device described in U.S. Pat. No. 4,157,085 has problems. One problem is in the use of silicone as the envelope material. In thicknesses that are adequate for product safety, silicone is relatively impermeable to extracellular water. In order to compensate for the required silicone thickness and allow for timely inflation of the tissue, the implant must contain an inordinately high amount of solute. Another, related problem is that rupture of the implanted envelope results in dispersion of highly concentrated solute into the surrounding tissues.
EP 0 784 987 discloses an inflatable prosthesis that contains a dehydrated substance which forms a gel when mixed with an aqueous solution. The dehydrated substance is a polyacrylamide, polyvinylpyrrolidone, hydroxypropyl methyl-cellulose, polyvinyl alcohol, polyethylene oxide, polypropylene oxide, polyethylene glycol, polylactic acid polyglycolic acid, hydrogel polyurethane, chrondotoin sulfate, hyaluronic acid, alginate, etc. The dehydrated substance is placed within an outer shell constructed from a polydimethyl siloxane, a polyurethane, a polyurethane/polyester copolymer, or another similar viscoelastic material. As shown in FIG. 3 of the publication, after the prosthesis is implanted into the body, an aqueous solution is added to the inner cavity thereof by a syringe. The addition of the aqueous solution combines with the dehydrated substance in the cavity to form a gel within the implant.
U.S. Pat. No. 5,756,632 claims an article comprising a hydrophilic gel with a water content of at least about 35% substantially enclosed within a membrane comprising a biocompatible, hydrophilic, segmented block polyurethane copolymer. The block copolymer comprises about 5 to 45 weight-% of at least one hard segment and about 95 to 55 weight-% of at least one soft segment. The soft segment of the block copolymer comprises at least one hydrophilic, hydrophobic, or amphipathic oligomer selected from the group consisting of aliphatic polyols, aliphatic polyamines, aromatic polyamines, and mixtures thereof. This patent also claims a method of permeating molecules—such as glucose, angiotensin I, glucagon, insulin, aprotinin, albumin, IgG, and oxygen—while substantially preventing the passage of cells and particulate matter between two fluids, using a biocompatible, hydrophilic, segmented block polyurethane copolymer.
U.S. Pat. No. 6,264,695 discloses a spinal nucleus implant for replacement of a portion of nucleus pulposus tissue removed from a spinal disc. The implant is an anisotropically swellable, biomimetic xerogel plastic, having a negatively charged lubricious surface and having a two phase structure with a hydrophobic phase having high crystallinity and low water content and a hydrophilic phase having low crystallinity and high water content. The xerogel plastic is capable of rehydration and of osmotic movement of liquid therethrough in response to osmotic pressure change to thereby increase and decrease liquid content in its hydrated state. Example 1 of the patent molds an acrylonitrile/acrylic acid block copolymer into the shape of a nucleus pulposus, and indicates that when the molded article is implanted into a vertebrate spine, it will be partially hydrated by body fluids, swelling to the shape and tension required for its long-term function. The patent also discloses surgical implant procedures utilizing the disclosed spinal nucleus implant.